The present invention relates to a method for preventing the bleeding of a plasticizer on the surface of shaped articles of plasticized polyvinyl chloride resins or, in particular, to a method for preventing bleeding of a plasticizer on the surface of shaped articles of polyvinyl chloride resins plasticized with a plasticizer having at least one aromatic nucleus in a molecule by the modification of the surface properties of the shaped articles.
Polyvinyl chloride resins are one of the most important classes of thermoplastic synthetic resins used for manufacturing various kinds of shaped articles useful in a very wide field of applications. Shaped articles of polyvinyl chloride resins are classified as a rigid type ones and a flexible type according to the absence or presence of a substantial amount of a plasticizer in the resin compositions. The rigidity or flexibility of shaped articles of a polyvinyl chloride resin can be controlled by the incorporation of a plasticizer such as a phthalic ester. Polyvinyl chloride resins thus plasticized are fabricated into various kinds of flexible articles including films, sheets, synthetic leathers, tubes, hoses, bags, packings, covering materials and the like directed to the uses in the fields of medical and sanitary wares, cooking wares, wrapping materials for foodstuffs, insulation of electric wires and cables, materials used in agricultural plant growing, materials for buildings and the like.
One of the most difficult problems involved in the shaped articles of plasticized polyvinyl chloride resins is the so-called bleeding of the plasticizer contained therein. Bleeding is a phenomenon by which the plasticizer contained in the body of the shaped article migrates with time toward the surface of the article and is lost by evaporation into the atmosphere or by transfer to another body in contact with the shaped article. This phenomenon of bleeding of the plasticizer, as well as of the other additives contained in the shaped articles detrimentally affects the appearance as well as various mechanical or physical properties and may result in a shortened serviceable life of the articles.
The phenomenon of bleeding is especially undesirable when the shaped article is to be used with a medical purpose such as a bag or container of blood for transfusion or to be used in contact with foodstuffs because of the toxicity of the plasticizer and other additives presenting limitations to the application of polyvinyl chloride resins.
Various attempts have been made hitherto to decrease the bleeding of plasticizers and other additives on the surface of articles utilizing the irradiation with ionizing radiations or ultraviolet light, corona discharge at a relatively high gas pressure, e.g. larger than 100 Torr, and treatment with chemicals. These prior art methods are to some extent effective for improving various surface properties of articles of polyvinyl chloride resins such as heat resistance, anti-solvent resistance, affinity with water, electrostatic charging, printability and the like. However, the effectiveness of these methods in preventing plasticizer bleeding is rather small. Moreover, it is sometimes unavoidable that advantageous properties of the shaped articles of polyvinyl chloride resins are adversely affected by these methods.
For example, the irradiation with an ionizing radiation produces crosslinks in the surface layer as well as in the body of the article. The high energy of the radiation results resulting in undesirable changes in the mechanical properties of the plasticized polyvinyl chloride resins. Irradiation with ultraviolet light is adversely affects in the coloring of the surface due to the oxidative degradation taking place in the surface layer of the articles. The treatment with corona discharge is not practicable due to the large difficulty in the process control owing to the rather unstable nature inherent to the discharge. Further, chemical means are not free from the problems of erosion by the chemicals or poor adhesiveness or durability of the coating compositions.